The invention relates to charge-coupled devices, and more especially to charge-coupled devices used for imaging and having two readout registers.
Charge-coupled devices (CCD's), in which a matrix or array of charge "pockets" is formed by the incidence of light upon the device, are well known and are often used in imaging systems. A typical application is the conversion of optical images into electrical signals in a TV system, in which context the CCD operates as a substitute for the already well-established vidicon tube.
FIG. 1 shows a typical CCD array utilising what is known as "frame transfer". The device 10 consists of two sections: an image section 20 and a line readout section 30, both of which are disposed on a common substrate. The image section 20 is made up of a number (in this case, 12) of horizontal electrodes 22 mounted on a substrate (not shown). The electrodes 22 are isolated from each other, are insulated from the substrate below by an oxide layer and run along the entire width of the array. To allow the CCD array to be light-sensitive, the electrodes are fabricated from a material that is semi-transparent to light, namely poly crystalline silicon (polysilicon).
Running vertically down the image section 20 are a number of channel stops 24. These are electrically inactive, doped (e.g. p-type) regions in the substrate and serve to constrain any charge produced in the substrate by the incidence of light upon the device to a number of "potential wells" bounded effectively by the width w of an electrode and the distance d between adjacent channel stops; one such "well" is shown shaded at 26. Given now the potential conditions on the 3-phase clock lines 23 connected to the electrodes, as shown in FIG. 1, charge 25 will be set up in each of those wells situated below an electrode having a positive potential with respect to the substrate. The amount of charge (negative) in each well will be proportional to the amount of light falling upon that well. The CCD image area is effectively a pixel array, whose dimensions are, in this case, 5 pixels wide.times.4 pixels high.
Once sufficient exposure to light falling onto the array from the image has been allowed (so-called "integration time"), the contents of image section 20 are clocked line by line, under the control of suitably changing potentials on clocking lines 23, out of the image section and into line readout section 30, where they are serially clocked out of the device at output amplifier 32, using clock lines 34.
In the device of FIG. 1 only one readout section or register is used. Indeed, most uses of the CCD in such imaging systems require only one readout register, through which the charge contents of the image array are sequentially output to an amplifier and whatever further processing stages are desired. However, there are applications which demand the use of two registers situated at opposite ends of the CCD image area, through either of which the image contents may be read at will. It is, for example, sometimes necessary to be able to read sequential image frames through two outputs alternately.
A second application lies in the use of a so-called "time delay integration" technique, the basis of which is the scanning of the field of view of the CCD array across an image, for example by means of a rotating mirror. Whereas, however, in a conventional imaging system of the scanning type each line of the image is sequentially detected by a one-dimensional array of detectors (corresponding to one of the rows or columns of charge pockets shown in FIG. 1), in the present application the scanned image is allowed to fall onto a two-dimensional CCD array, as shown in FIG. 2. In addition the clocking of the array is arranged to synchronise with the scanning rate, so that as one particular line of the image moves across the array its position is tracked by the clocking taking place in the same direction. The effect of this technique is to lengthen the integration time of each line of the image scanned and thereby to increase the imaging sensitivity.
It can be desirable in imagers using this particular imaging technique to have the capacity for movement of an image line across the CCD array to take place in either of two opposite directions, and it is therefore clearly desirable to be able to switch readily between the two readout registers to cater for this. The arrows in FIG. 2 indicate that the charge contents of the image section 40 may be read out either through register 42 or through register 44, emerging at outputs 1 and 2 respectively.
Hitherto such applications involving two readout registers have required the use of two output amplifiers on the CCD, one for each register. The drawback with this approach, however, has been the unequal treatment of image information from the two registers, due to the inability to perfectly match the two amplifiers in terms of gain and offset, etc. There is also an increased hardware cost.